1. Field of the Invention
The present invention relates to semiconductor device manufacturing, and more specifically, to a photomask, a method of controlling the focus condition or the exposure dose condition of an exposure system for use in a photolithography process using the photomask, and a semiconductor device manufacturing method using the controlling method.
2. Description of the Related Art
With the trend toward finer device patterns, tolerances for process conditions (process margins), such as the depth of focus, the exposure dose adjustment, and the like of an exposure system, in a photolithography process have been decreased. Accordingly, what is desired is a high accuracy controlling (monitoring) method for an exposure dose condition and the like minimizing error factors which consume process margins.
Heretofore, a focus condition has been controlled by measuring the length l (diagonal main axis length) of a rhombic pattern 1010 as shown in FIG. 1B, which is transferred on a wafer using a photomask provided with a rhombic monitor pattern 1000 having a diagonal minor axis length of approximately 0.5 μm, for example, as shown in FIG. 1A.
FIG. 2 shows the relationship between a deviation distance from a focus position (defocus value) and the length l of the rhombic pattern 1010 transferred on a wafer. As the rhombic pattern 1010 approaches the best focus, the resolution of the rhombic pattern 1010 is getting higher, and a finer edge portion thereof can be transferred on a wafer. Consequently, the length l of the rhombic pattern 1010 reaches the maximum value at the position of the best focus and decreases with increasing defocus value. Accordingly, when semiconductor device manufacturing is performed, before a product lot is released into a manufacturing process, the relationship between the defocus value and the length l of the rhombic pattern 1010 transferred on a wafer is determined, and the best focus condition in the exposure process for the product lot are decided.
In the case where the focus condition for the product lot is controlled, a transferred pattern is fabricated using a photomask provided with the rhombic pattern 1000 under the same exposure condition as that for the product lot, and the length l of the transferred rhombic pattern 1010 is measured, thus monitoring the defocus value.
However, in a heretofore-known focus monitoring method using the above-described rhombic pattern, it is impossible to obtain information as to whether the focus deviation of an exposure system has been in an upward or downward direction. Moreover, the size and shape of a monitor pattern are significantly different from those of a device pattern. Accordingly, when the variations in an exposure dose and the like occur, the sensitivity of the monitor pattern to these variations is different from that of the device pattern, which sometimes reduces the reliability for monitoring.
A method of monitoring focus without being influenced by exposure dose includes a method of detecting the variation amount of focus as a positional deviation amount of patterns, which is disclosed in U.S. Pat. No. 5,300,786. However, in this method, since the detection sensitivity for focus largely depends on the shape (σ) of a light source, a sufficient sensitivity can be obtained under an exposure condition in which σ is relatively low, but a sufficient sensitivity cannot be obtained under a generally used condition in which σ is relatively high or under a condition in which annular illumination is used.
Moreover, an exposure dose monitoring method is disclosed in Japanese Unexamined Patent Publication No. 2000-310850. In this method, special patterns are used for monitoring exposure dose, and either the dimensions of the patterns transferred on a wafer or the deviation between the center positions of the patterns is measured, thus making it possible to monitor exposure dose. However, also in this case, a device pattern and an exposure dose monitor pattern have completely different dimensions and shapes. Accordingly, when both of these patterns have different sensitivities to the exposure dose, it cannot be said that the exposure dose is accurately monitored for the device pattern.